1. Field of the Invention
The present invention relates to an antenna device, in particular an antenna device, which is suitable for communication in a microwave range (3 to 30 GHz) and a millimeter wave range (30 to 300 GHz) used for communication, distance measuring equipment or broadcast.
2. Discussion of Background
Heretofore, a disc monopole antenna, which is disclosed in M. Hammoud et al, “Matching The Input Impedance of A Broadband Disc Monopole”, Electron. Lett., Vol. 29, No. 4, pp. 406–407, 1993, has been known as an antenna having an operating frequency band in a wide band. FIG. 24 is a schematic view showing this disc monopole antenna. This disc monopole antenna is configured to include a planar monopole 201 connected to a coaxial line 202. Specifically, the planar monopole 201 is disposed as to be upright with respect to a metal plate 203 at a position away from the metal plate 203 by a distance L. It is possible to provide optimum matching so as to have a desired characteristic by adjusting the distance L.
An antenna, which is shown in FIG. 25 and is disclosed in Japanese Patent No. 3273463, has also been known. This antenna is configured so that a pair of radiating plates 211a and 211b having a substantially semi-circular shape are disposed. The radiating plates 211a and 211b are formed by removing substantially semi-circular portions from two conductive plates having a semi-circular shape, the substantially semi-circular portions being smaller than the conductive plates and being concentric with the corresponding conductive plate. Each of the radiating plates 211a and 211b has a substantially semi-circular cut-out portion 241a or 241b concentrically formed in a central portion of a concentric circle of the semi-circular shape. The two radiating plates 211a and 211b are disposed so as to have the apexes 221a and 221b of the respective circular arcs confronting each other whereby power is fed between the apexes 221a and 221b of the radiating plates 211a and 211b. A coaxial cable 231 is disposed along the centerline Ox of the radiating plate 211b. 
An antenna 253, which is configured to have a semi-circular radiating conductor 251 printed on a ceramic plate 250 and to have a feed point disposed for connection between a signal line and an edge portion 252 of a semicircular shape of the radiating conductor 251 as shown in FIG. 26, is also disclosed in Do-Hoon Kwon, Yongjin Kim et al, “A Small Ceramic Chip Antenna for Ultra-Wideband Systems”, UWBST & IWUWBS 2004 Conference Proceedings, TA4-3, pp. 307–311, 2004. The radiating conductor 251 has a narrow slit 254 formed in the vicinity of the edge portion 252 so as to be capable of adjusting an antenna characteristic. This non-patent document states that this arrangement can realize an antenna having an operating frequency band in a wide band.
An antenna 263, which is shown in FIG. 27 and is disclosed in US-A-2004-0100408, has been also known. This antenna comprises a rectangular radiating conductor 264 disposed on a dielectric member 261, and a ground conductor 262 so as to serve as a monopole antenna.
By the way, the antenna shown in FIG. 24 is a monopole antenna. This antenna is configured to include a radiating element comprising the planar disc monopole 201 and a ground conductor comprising the metal plate 203. The radiating element and the ground conductor are disposed so as to be perpendicular and orthogonal with each other. Accordingly, the radiating element is disposed to be upright with respect to the ground conductor so as to have a three-dimensional configuration, occupying a three-dimensional space as an antenna having a three-dimensional structure. Since the metal plate 203 needs to have about 10 times larger size than the diameter of the radiating conductor forming the planar disc monopole 201 in this configuration, the metal plate has a larger shape of, e.g., 300 mm×300 mm. Thus, the antenna shown in FIG. 24 has such a three-dimensional structure, and the ground conductor also has a large shape. As a result, the antenna shown in FIG. 24 is not suitable for a compact antenna.
In the antenna shown in FIG. 25, when each of the radiating plates 211a and 211b is formed in a semi-circular shape having a diameter of 150 mm, the lower limit frequency, which has a VSWR (Voltage Standing Wave Ratio) of substantially 2 or below, is 600 MHz. The wavelength λ at this lower limit frequency (600 MHz) is substantially 500 mm. This means that the diameter of the radiating plates 211a and 211b needs to have a length equal to at least substantially 0.3 time a wavelength in an operating frequency band used in a radio wave. Since the semi-circular radiating plates need to have a diameter having a length equal to at least substantially 0.3 time a wavelength as stated earlier, the outline of the antenna device needs to have a large occupied area, i.e., an antenna area of substantially 0.3 time a wavelength× substantially 0.3 time a wavelength. When an attempt is made to reduce the lower limit frequency of the operating frequency band, the outline of the antenna device needs to be made larger. Accordingly, the antenna shown in FIG. 25 is not suitable for a compact antenna.
Additionally, it is difficult to perform impedance adjustment since power is fed from the coaxial cable 231 to the apexes 221a and 221b. Accordingly, the antenna shown in FIG. 24 is not an antenna having a high degree of freedom in design.
On the other hand, the antenna 253 shown in FIG. 26 is also a monopole antenna. Accordingly, this antenna needs to have a ground conductor (not shown) in order to serve as an antenna. When the radiating conductor 251 has a semi-circular shape having a diameter of 10 mm, it is required that the ground conductor have a rectangular shape of 30 mm×30 mm and that the antenna 253 has an outline formed in a rectangular shape of 40 mm×30 mm. The lower limit frequency that the antenna thus configured has a VSWR of substantially 2.3 or below is 3.1 GHz. Accordingly, the antenna 253 having such an outline of 40 mm×30 mm needs to have an area of substantially 0.4 time a wavelength×substantially 0.3 time a wavelength with respect to the wavelength of the lower limit frequency of 3.1 GHz. When an attempt is made to reduce the lower limit frequency in order to expand the width of the operating frequency band, the outline of the antenna device needs to be made larger. Accordingly, the antenna shown in FIG. 26 is not suitable for a compact antenna.
Additionally, it is impossible to provide the antenna shown in FIG. 26 as an antenna capable of reducing the lower limit frequency of the operating frequency band and having a high degree of freedom in design since the radiating conductor 251 is fixed in such a semi-circular shape.
Additionally, the antenna shown in FIG. 27 is also a monopole antenna and needs to have a ground conductor. When the radiating conductor has dimensions of 8 mm×10 mm, it is required that the ground conductor have a rectangular shape of 20 mm×35 mm and that the antenna 263 have an outline formed in a rectangular shape of 28 mm×45 mm. The lower limit frequency that the antenna thus configured has a VSWR of substantially 2 or below is 3 GHz. Accordingly, the antenna 263 having such an outline of 28 mm×45 mm needs to have an area of substantially 0.28 time a wavelength×substantially 0.45 time a wavelength with respect to the wavelength of the lower limit frequency of 3.1 GHz. When an attempt is made to reduce the lower limit frequency in order to expand the width of the operating frequency band, the outline of the antenna needs to be made larger to increase the occupied area of the antenna 263. Accordingly, the antenna shown in FIG. 27 is not suitable for a compact antenna.